Synchronous motors having inwardly projecting stator poles and a permanent magnet rotor are well known and are made in a variety of constructions. In order to have a self-starting motor with a permanent magnet rotor, it is necessary to keep the peripheral velocity of the rotor and the moment of inertia of the rotor small. Because of this, such motors are constructed exclusively as multiple-pole motors or motors having a small diameter.
One prior art approach to such synchronous motor construction employs a single stator structure, as shown in FIG. 6. In this figure, the multiple-pole rotor is designated as 20, the multiple-pole stator surrounding the rotor is designated as 21 and the inwardly projecting poles of the stator are designated as 22. Stator pole windings of different phases, w and w', are alternately disposed on the respective stator poles 22. The center-to-center pole spacing .tau.pr of the permanent magnet rotor must be made twice as large as that of the center-to-center pole spacing of the stator .tau.ps for a predetermined stator and rotor diameter. In this construction, the rotor assumes a disadvantageous rest position for self-starting purposes. While such single stator-type synchronous motors do have low manufacturing costs, they have not proved useful in practice, since the excitation power required for motor startup must be made much too large. This, in turn, results in an impermissible thermal overload of the stator windings and demagnetization of the permanent type magnet rotors results.
A considerable improvement of the thermal and electromagnetic properties of the synchronous motor has been accomplished by employing a construction where two single-phase systems are mechanically coupled and axially disposed in series. Such a construction is shown in FIGS. 7a and 7b. Again, in this figure, 20 represents the rotor of the motor I, 21 represents the stator of that motor, and 22 represents the inwardly projecting poles (corresponding prime numbers refer to similar elements of motor II). Note that all of the stator poles of motor I are excited by one phase while all of the stator poles of II are excited by the second phase. The manufacturing costs of synchronous motors constructed in this fashion are, however, considerably greater than those of a motor having a single stator.
Attempts have been made, therefore, to combine the favorable features of both motor systems without having to accept their poor or disadvantageous aspects. For this purpose, the stator pole for both windings w and w', having respective excitation fluxes .PHI..sub.1 and .PHI..sub.2, were not symmetrically disposed on the inner stator circumference about a diameter, but were grouped together in stator-pole groups SG and SG' per phase, as shown in FIG. 8. In this figure, 20 is the rotor, 21 the stator and 22 the inwardly projecting pole. The center-to-center interpole spacing of the rotor and the spacing within the stator pole groups has been set equal to .tau.p.
Stator-pole groups of the individual phases were displaced on the inner circumference of the stator by a spatial displacement angle .delta. = .tau.p/m, which displacement angle is required for a rotating field (wherein m denotes the number of phases). FIG. 8a illustrates the excitation circuitry and the associated electrical displacement of the phases.
The solution thus derived for a synchronous motor having a single stator had, for example, in an eight-pole, two-phase machine with the pole pairs p = 4, the number of exciting coils reduced from 16 to 6. In this arrangement, only a single permanent magnet was required for the rotor and the awkward pairing of the rotor magnets in the FIG. 7 embodiment was dispensed with. This resulted in a very significant reduction in manufacturing costs. A patent application for this motor principle has been filed and published as German Patent Application No. 2,337,905.
A volumetric comparison of the efficiency and power output of the motor described in FIG. 8 with the motor of FIG. 7, shows that the single-stator type synchronous motor of FIG. 8 has reduced power and efficiency. It is believed that this inefficiency is a result of poor utilization of the available stator space and of the permanent magnet.